The main objective of the study proposed is the isolation and characterization of T-cell factors which replace T-cells and specifically cooperate with B-cells in production of antibodies. These factors will be isolated from educated T-cell supernatant by affinity chromatography on antigen columns. The activity of the factors will be assayed in vivo and in vitro by measuring their helper effect on B cells. Antisera to the isolated factors will be prepared and used for assay and for the isolation of cross-reacting components on T or B cells as well as in T cell supernatant. It will be attempted to characterize the molecular properties of the T cell factors and to compare them with other molecules involved in the immune system such as antibodies, H-2 or Ia antigens. This study will provide a unique opportunity to approach the understanding of the T cell receptors and to analyze their function in triggering B cells for antibody production or in cell mediated immunity.